Add external/conscrypt to visibility

`conscrypt` has a `required` property set to boringssl_self_test.
Add the package where `conscrypt` is located to the visibility property
of the test module.

This is in preparation for an upcoming change in Soong which handles the
installation of required modules.

Bug: 321626681
Test: builds

Change-Id: Ie97f937d7408f0221e73ae3843a23bce5610817a
1 file changed